1. Field of the Invention
The invention relates to stapling devices in general and particularly, to electrical staplers adapted for use with an electrophotographic device. Such staplers include a force generating device which is automatically actuated to drive a staple into a stack of sheets in order to fasten them together.
2. Prior Art
Staplers and their operation are well known in the art. Electrical staplers are more complex than the manual staplers and they include essentially the same mechanical components, i.e., an anvil or clinching device and a driver assembly. The actuating force for driving the staples is usually provided by a solenoid.
By way of example, U.S. Pat. No. 3,278,101 is a prior art stand-alone electrical stapler. In the illustrated stapler, electromechanical force powers the driver. The stapler includes a sheet receiving receptacle disposed between the anvil and driver assembly. A microswitch is disposed relative to the receptacle. Sheets to be stapled are placed in the receptacle and activate the microswitch to charge a capacitor. The capacitor charge current momentarily operates a relay having a contact that activates an electromagnet which generates a an impulse force for actuating the driver which forces a preformed staple into the sheets. The patent shows that, for force control, a capacitor with variable capacitance or an electromagnet with circuit taps may be provided to adjust the electromagnetic force produced in the electromagnet to drive the staple with a force depending upon the load on the staple. For example, the load may result from thickness of the sheets to be fastened or the number of sheets. The patent does not, however, teach how the stated characteristics, i.e., the load, relative to the sheets can be sensed and utilized to adjust the driving force.
Another type of prior art electrical stapler is primarily adapted for use with copiers. Such staplers are disposed in the copy sheet output paper path. Sets of copy sheets are formed and stapled together by the stapler. U.S. Pat. No. 4,187,969 is an example of copier-related prior art staplers. The stapler includes a housing with a passageway for storing preformed staples therein. A stationary clamping surface is connected to the passageway. A movable support surface is disposed in spaced alignment with the clamping surface. Means are provided for delivering a stack of sheets to the support surface.
In operation, the housing member with the clamping surface is held stationary. The movable support surface brings the stack of sheets into contact with the clamping surface. With the stack of sheets securely held against the clamping surface, a pneumatic operated device drives the staple into the stack.
Another type of electrical stapler primarily adapted for use with electrophotographic printers does not use preformed staples. This type of stapler is fitted with a staple forming mechanism. Usually the staple forming mechanism includes a device for cutting a predetermined length of wire from a wire supply spool. The wire is next formed into a staple. The staple is then transported into a supply magazine where it is driven into a stack of sheets. A clinching mechanism then moves into place and clinches the ends of the wires. U.S. Pat. No. 4,134,672 is an example of the last mentioned type of electrical stapler.
Often a stapler is required to bind stacks of sheets or documents of variable thickness. Variable thickness stacks usually present two problems for staplers, namely, the appropriate length of the stroke and the force which must be applied to the driving element. The length of the stroke is critical to the operation of the stapler in that if the driving element does not travel the full length of the stroke, the staple will not be ejected from the head. Similarly, if the force is not sufficient, the staple will not be driven through the stack.
The attempt of the prior art to solve the problem falls into two categories. In the first, the driving force used to drive the staple is more than is necessary. The philosophy is that by using a relatively high impact force it is assured that the staple will go through the stack. Alternately, the stapler is manually adjusted whenever the thickness of a stack changes.
Neither of these approaches satisfactory solves the variable thickness problem. The manual adjustment is unacceptable because the factor, such as paper weight, etc., which affects the stack thickness is dynamic in nature and cannot be solved by static approach in copiers. Likewise, the use of excessive force tends to waste energy and to increase the overall cost of the stapler.